1. Field of the Invention
The invention generally relates to a liquid crystal display, more particularly to a liquid crystal display having enhanced display quality and capable of improving response time characteristics.
2. Description of the Related Art
An in plane switching(hereinafter xe2x80x9cIPSxe2x80x9d) mode liquid crystal display is a device for driving a horizontal electric field developed by Hitachi, Japan in order to compensate viewing angle characteristics of a twisted nematic(hereinafter xe2x80x9cTNxe2x80x9d) mode liquid crystal display.
FIG. 1 is a plane view showing certain part of a sub pixel of the IPS mode liquid crystal display. As shown in FIG. 1, electrode 2a,2b for driving liquid crystal molecules are disposed with a selected distance on the same substrate, for example a lower substrate 1. Herein, the distance between the electrodes 2a,2b is selected such that an electric field is parallel with a surface of the substrate 1. A homogeneous alignment layer(not shown) is formed on the surface of the substrate 1 in which the electrodes 2a,2b are formed. The homogeneous alignment layer is rubbed to make a selected angle, for example approximately xc2x145xc2x0 with the electrodes 2a,2b. Furthermore, a first polarizing plate(not shown) having a polarizing axis coincided with the rubbing direction is provided at backside of the lower substrate 1. Meanwhile, an upper substrate(not shown) is arranged opposite to upper portion of the lower substrate 1 with a selected distance. The homogeneous alignment layer is formed at an inner surface of the upper substrate and a second polarizing plate(not shown) perpendicular to the first polarizing plate is formed at an outer surface thereof. A liquid crystal layer(not shown) having liquid crystal molecules 3 is interposed between the lower substrate 1 and the upper substrate(not shown).
Operation of the IPS mode liquid crystal display is as follows.
First of all, when no voltage is applied to the electrodes 2a,2b, long axes of the liquid crystal molecule 3 are disposed parallel with the rubbing axis of the alignment layer. At this time, the polarizing axis of the first polarizing plate is parallel with the rubbing direction, and the polarizing axis of the second polarizing plate is perpendicular to the rubbing direction. Therefore, an incident light transmitted from the backside of the lower substrate 1 can pass the first polarizing plate and the liquid crystal layer, but does not pass the second polarizing plate thereby showing a dark state in the screen.
On the other hand, when a selected voltage is applied to the electrodes 2a,2b, there is formed an electric field E between the electrodes 2a,2b. Herein, since those electrodes 2a,2b are formed at the same substrate surface, the electric field E is parallel to the substrate surface. Therefore, in case of positive dielectric anisotropy, the liquid crystal molecules arranged in a rubbing direction of the alignment layer are twisted along the clockwise direction so that the long axes of the liquid crystal molecules are parallel with the electric field. As a result, the incident light transmitted from the backside of the lower substrate passes through the first polarizing plate, the liquid crystal display and the second polarizing plate since the arrangement of the liquid crystal molecules is changed, thereby showing a white state in the screen.
As known in the art, the refractive anisotropy(or birefringence, xcex94n) is occurred due to the difference of the lengths of the long and the short axes. The refractive anisotropy xcex94n is also varied from the viewer""s viewing directions. Therefore a predetermined color is appeared on the region where the polar angle is of 0 degree and the azimuth angle range of degrees 0, 90, 180 and 270 in spite of the white state. This phenomenon is regarded as color shift and more detailed description thereof is attached with reference to the equation 1.
T≈T0 sin2(2"khgr")xc2x7sin2(xcfx80xc2x7xcex94nd/xcex)xe2x80x83xe2x80x83equation 1
wherein,
T: transmittance;
T0: transmittance to the reference light;
"khgr": angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate;
xcex94n: birefringence;
d: distance or gap between the upper and lower substrates(thickness of the liquid crystal layer); and
xcex: wavelength of the incident light.
So as to obtain the maximum transmittance T, the "khgr" should be xcfx80/4 or the xcex94nd/xcex should be xcfx80/2 according to the equation 1. As the xcex94nd varies with the birefringence difference of the liquid crystal molecules from viewing directions, the value of xcex is varied in order to satisfy xcfx80/2. According to this condition, the color corresponding to the varied wavelength xcex appears.
Accordingly, as the value of xcex94n relatively decreases at viewing direction xe2x80x9cxxe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases also. Consequently a color of blue having a shorter wavelength than that of a color of white appears.
On the other hand, as the value of xcex94n relatively increases at a viewing direction xe2x80x9cyxe2x80x9d toward the long axes of the liquid crystal molecules, the wavelength of an incident light relatively increases also.
Consequently a color of yellow having a longer wavelength than that of the color of white appears.
Deterioration is caused in the picture quality of IPS mode liquid crystal display.
Furthermore, since no electrode is formed at the upper substrate of the foregoing IPS mode liquid crystal display, the response time thereof is slower than that of the TN mode liquid crystal display, and it is also difficult to remove the static electricity generated at the upper substrate.
Accordingly, one object of the present invention is to provide a liquid crystal display capable of improving picture quality by preventing color shift.
Further, the other object of the invention is to provide a liquid crystal display capable of improving response time characteristics.
One another object of the invention is to provide a liquid crystal display capable of easily discharging the static electricity generated at an upper substrate of the same.
In order to accomplish these objects, the invention provides a liquid crystal display comprises:
an upper substrate and a lower substrate opposed to each other with a selected distance;
a liquid crystal layer sandwiched between the upper and the lower substrate, and having a plurality of liquid crystal molecules;
a pixel electrode formed at an inner surface of the lower substrate, wherein a display signal is applied to the pixel electrode; and
a counter electrode formed an inner surface of the upper substrate and forming an electric field together with the pixel electrode by applying a common signal, thereby driving the liquid crystal molecules;
wherein a voltage difference is occurred between the counter and the pixel electrodes, there are simultaneously formed an electric field disposed vertical to substrate surface and an oblique electric field disposed oblique to substrate surface between the pixel electrode and the counter electrode.
Furthermore, the invention provides a liquid crystal display comprising:
an upper substrate and a lower substrate opposed to each other with a selected distance, in which sub pixels are defined respectively;
a liquid crystal layer sandwiched between the upper and the lower substrates, and having a plurality of liquid crystal molecules having negative dielectric anisotropy;
a pixel electrode formed at an inner surface of the lower substrate and made of a transparent material, wherein a display signal is applied to the pixel electrode; and
a counter electrode formed at an inner surface of the upper substrate, made of a transparent material and forming an electric field together with the pixel electrode by applying a common signal, thereby driving the liquid crystal molecules;
wherein no voltage difference is occurred between the counter and the pixel electrodes, long axes of the liquid crystal molecules are aligned vertical to substrate surface;
wherein a voltage difference is occurred between the counter and the pixel electrodes, there are simultaneously formed an electric field disposed vertical to substrate surface and an oblique electric field disposed oblique to substrate surface between the pixel electrode and the counter electrodes.
The invention still provides a liquid crystal display comprising:
an upper substrate and a lower substrate opposed to each other with a selected distance;
gate bus lines and data bus lines arranged in a matrix type and defining sub pixels;
a storage electrode disposed at each sub pixel of the lower substrate;
a pixel electrode overlapped with a selected portion of the storage electrode, disposed at each sub pixel of the lower substrate and made of a transparent conductive material;
a counter electrode disposed at each sub pixel of the upper substrate and made of a transparent conductive material; and
homeotropic alignment layers disposed at a surface of the pixel electrode and the lower substrate, and at a surface of the counter electrode and the upper substrate,
wherein the storage electrode comprises a first region extended along a width direction of the sub pixel and dividing the sub pixel region in two; a second region disposed parallel with the first region and adjacent to the corresponding gate bus lines; and a third region connecting the first and the second regions and disposed adjacent to the corresponding data bus line,
wherein the pixel electrode comprises a body overlapped with the third region of the storage electrode and extended along a length direction of the sub pixel; a protruding part extended from the body to a space between the first region and the second region; and a plurality of teeth of comb formed at one side of the protruding part and at the sub pixel region of one side of the protruding part, and extended parallel with the first region of the storage electrode from the body,
wherein the counter electrode further comprises a first electrode part formed along a length direction of the sub pixel to correspond with the body of the pixel electrode; a second electrode part to correspond with the protruding part of the pixel electrode and extended from the first electrode part to occupy certain region of the sub pixel; and a third electrode part extended from the first and the second electrode parts and disposed on the upper substrate to correspond with spaces between the teeth of comb of the pixel electrode.